Solvent composition regulates the Se : Sb ratio in antimony selenide nanowires deposited from thiol-amine solvent mixtures.

Antimony selenide (Sb2Se3), a V2VI3 semiconductor with an intriguing crystal structure, has demonstrated improved power conversion and solar-to-hydrogen efficiencies in recent years. Depositing antimony selenide nanowires (NWs) from a solution such as a thiol : amine "alkahest" ink is a low-cost and facile route to deposit high surface area photocathodes. However, little is known about the correlations between the solvent composition and the crystallites' structure and optoelectronic properties, which are crucial for photovoltaic and photoelectrochemical applications. We found that the Se : Sb ratio in the NWs decreases from 3 : 2 to less than 1 : 1 with decreasing thiol : amine ratio in the ink used for deposition but not in the solvent mixture used for dissolving the metals. The reduced Se : Sb ratio in the solid NWS correlates with an optical bandgap wider by ∼0.3 eV in comparison to stoichiometric NWs, a decrease of the NWs diameter from 180 to 30 nanometers, and a ∼0.2 eV larger work function. In addition, we found that the Se : Sb ratio is not uniform along the NWs, which causes a surface potential increase near the tips of the NWs due to a lower Se : Sb ratio near the NWs tips. The increased surface potential near the tips corresponds to a driving force, due to doping or graded bandgap broadening, that facilitates the migration of photoexcited electrons towards the NW tips. Our findings unlock a path for fine-tuning the optoelectronic properties of antimony selenide towards improving the performance of antimony selenide solar cells and photocathodes.

Chemistry and Charge Trapping at the Interface of Silver and Ultrathin Layers of Zinc Oxide.

Zinc oxide, a wide-band-gap semiconductor, shows intriguing optoelectronic properties when coupled with Ag. Specifically, an absorbance band in the visible range that is not apparent in the separated materials emerges when the interface is formed. Interestingly, photoexcitation of this "interface band" or band-to-band results in a counterintuitive photovoltaic response when a supra/sub-band-gap light is shone. To investigate the origin of this absorbance band and photovoltaic response, we studied in detail the energy-band alignment of ultrathin layers of ZnO (3-60 nm) with Ag. Our analysis indicated that an 'electrostatic potential cliff' is formed within the first 1-2 nm of ZnO. In addition, oxygen vacancies, presumably generated by AgxO-Zn bonds, form mid-gap acceptor states within these first few nm. Both effects facilitate a valence band-to-defect state optical transition that is confined to the interface region. The second type of defects-hole-trap states associated with zinc hydroxide-are spread throughout the ZnO layer and dominate the supra-band-gap photovoltaic response. These findings have potential implications in emerging technologies such as photocatalytic Ag/ZnO heterostructures that will utilize the long-lived charges for chemical work or other optoelectronic applications.